1. Field of the Invention
The present invention relates to a liquid ejection head, in particular, adopting a so-called fill-before-fire method for ejecting liquid, and also to a driving method of the head.
2. Description of Related Art
A liquid ejection head typified by an inkjet head adopting an inkjet system has a passage unit on which nozzles are formed for ejecting liquid, as disclosed in Japanese Patent Unexamined Publication No. 2003-39673. In the passage unit of the liquid ejection head, there are formed common liquid chambers for supplying liquid to the nozzles; and individual liquid passages leading from the common liquid chambers to the respective nozzles. A pressure chamber is formed in the middle of each individual liquid passage. An actuator is provided over the pressure chamber to apply pressure to the liquid in the pressure chamber. In controlling the liquid ejection head, the actuator is driven to apply pressure to the liquid in the pressure chamber so that liquid is ejected from the corresponding nozzle.
In a driving method of the actuator, pressure is applied twice to the liquid in the pressure chamber to eject liquid from the nozzle. For example, first, the volume of the pressure chamber is increased to decrease the pressure in the pressure chamber. Next, when a predetermined time has elapsed after the volume of the pressure chamber was increased, the volume of the pressure chamber is restored to the original volume to increase the pressure in the pressure chamber. Thereby, a proper intensity of pressure is applied to the liquid in the pressure chamber at a proper timing to eject liquid from the nozzle.
In the driving method of the actuator, a relation between the timing of restoring the volume of the pressure chamber to increase the pressure of the liquid in the pressure chamber, and the velocity of liquid ejected from the nozzle, is, for example, as shown in FIG. 9. In FIG. 9, the axis of abscissas corresponds to the timing, and the axis of ordinate represents the velocity of ejected liquid. As shown by a curved line 70 in FIG. 9, the liquid velocity to the timing ideally forms a smooth curve being convex upward, on which the timing at which the ejection velocity becomes the maximum is uniquely determined. Driving the actuator to apply a pressure at the timing at which the ejection velocity becomes the maximum, brings about efficient liquid ejection. An ink ejection method in which control is performed so as to apply the second pressure at the timing at which the ejection velocity takes the peak, is called fill-before-fire method.
The reason why such a peak appears on the ejection velocity to the timing of applying pressure, is as follows. When the pressure in the pressure chamber is decreased, a proper oscillation is generated in the individual liquid passage. When a pressure is then applied to the liquid in the pressure chamber, a pressure wave thereby generated is superimposed on the proper oscillation. Therefore, when the timing of applying the second pressure coincides with a peak of the proper oscillation, the velocity of liquid ejected from the nozzle becomes the maximum. On the other hand, when the second pressure is applied at a timing shifted from the peak of the proper oscillation, the liquid velocity decreases from the maximum value. The larger the difference of the timing of applying the second pressure from the timing that brings about the maximum velocity, the lower the velocity of liquid ejected from the nozzle. For the above reason, the curved line as shown in FIG. 9 is obtained.
On the other hand, when liquid is ejected from the nozzle by using the fill-before-fire method, problems may arise that ejection characteristics such as the velocity and quantity of liquid ejected from the nozzle become bad, and the ejection characteristics vary.